1. Field of the Invention
The present invention relates to video graphics and more particularly to a system and method for processing analog calligraphic video signals to convert them into digital data suitable for presenting stroke images on a raster display device such as a flat panel LCD display.
2. Problem to be Solved
Calligraphic video signals consist of a series of analog voltage signals indicative of X, Y, and Z components which may be used to produce an illuminated track or stroke on a display surface, such as the phosphor screen of a cathode ray tube (CRT). In a CRT, an electron beam illuminates the phosphors and in an XYZ CRT monitor the calligraphic signals are usually encoded for direct use in controlling the beam. Each X component is indicative of an X-deflection of the beam, i.e., a horizontal position on the screen, while each Y component indicates a Y-deflection or a vertical position on the screen. Each Z component is indicative of the brightness to be produced by the beam at the position defined by its coordinated X and Y components. A stroke image is thus produced by appropriately controlling the successive X, Y, and Z component combinations to produce an illuminated track on the screen.
While a CRT monitor can use the analog calligraphic signals directly, video displays typically use a raster scan system and XYZ calligraphic analog voltage signals cannot be used by a raster system without conversion. In a raster display, to form an image the beam is regularly swept from side to side across the surface of the screen in a raster action beginning at the upper left hand corner and proceeding in a series of successive lines to the bottom of the screen. The beam is then returned to the upper left hand corner to begin the next sweep. During the sweep, the Z component signals produce illuminations at successive positions in each line. Each illumination is indicative of the brightness to be produced by the beam at the respective positions defined by the Z component's coordinated X and Y components. Each image is thus formed by the series of illuminations along the prescribed raster lines in keeping with the successive X, Y, and Z component combinations. For such an operation it is convenient to use a frame buffer memory for storing the Z component signals digitally at addresses indicated by their respective X and Y coordinates. The contents of the buffer memory can then be used to appropriately control the illumination of the phosphors of the pixels of the display to create the succession of images defined by the video input signals.
In a typical video raster display, the beam after each sweep is returned at zero intensity to begin the sweep for the next image. In some applications, for example, in avionics displays, the intensity and the position of the beam have been controlled during the return to produce a form of overlay on the raster video image displayed. This overlay is what is referred to as a "stroke" or "stroke image" and is determined and controlled by analog voltage inputs of calligraphic video signals. Currently, in avionics applications flat panel LCD displays are replacing the conventional phosphor screen display with the attendant desirability of digital processing of the video signals since such displays conveniently use previously-mentioned frame buffers in the form of digital memories with each memory location in a buffer representing a location corresponding to a pixel on the display screen surface. The digital frame buffer contents are periodically transferred to produce a successive set of images on the display surface in accordance with successive sets of digital data supplied to the buffer. It is therefore necessary to convert the successive sets of calligraphic analog voltage signals to successive sets of digital data in the frame buffer, which data is transferred as pixel exciting signals to produce the distinctive illuminated strokes on the display for the overlay formation on the video images.
Although analog to digital conversion is normally straightforward and can be readily applied to video raster scan conversion, effective stroke scan conversion requires a different approach. Stroke symbology by its nature has a much higher resolution than conventional video and also must be presented with high quality anti-aliasing. Therefore, simple digitizing and storing of stroke symbology in a frame buffer would not only require a very large frame buffer to maintain the resolution, but additionally the scan conversion rate required to scale the stroke to an appropriate size to match the display size would be very high. Currently, memory components are not commercially available that can handle the requisite high speed. Further, other conversion artifacts must be considered, such as noise quantization, wherein small noise levels in the X and Y deflection signals can be magnified to full pixel position variation, and clock jitter quantization, wherein the apparent end points of the lines can vary by a pixel in a rhythmic manner due to beat frequencies between the display generator digital-to-analog (D/A) clock and the local analog-to-digital (A/D) clock.
3. Objects of the Invention
It is accordingly an object of the invention to convert XYZ calligraphic video signals to be presented as a stroke image on a raster display to digital data stored in a conventional frame buffer for the display.
It is a further object of the invention to provide a system and method for converting calligraphic video analog voltage signals to corresponding digital data capable of storage in a frame buffer from which it is used to produce high resolution graphic images on a raster display.
It is another object of the present invention to provide a system and method for digitally processing calligraphic video signals for suitable storage in a frame buffer and high resolution presentation on a raster display.
It is also an object of the invention to provide a system and method for converting calligraphic video analog voltage signals to corresponding digital data capable of producing high resolution, anti-aliased, stroke images on a flat panel LCD raster display.